Electrical analogue device



1 M- 3951 e. H. STEPHENSON 2,567,532

ELECTRICAL ANALOGUE DEVICE Filed Sept. 30, 1948 f'f ml) wen) GEOFFRE HU ON STEPHENSON By fiatented Sept. 11, 1951 ELECTRICAL ANALOGUE DEVICE Geoffrey Huson I Stephenson, London, England,

assignor to Electric & Musical Industries Limited, Hayes, England, a British company Application September 30, 1948, Serial No. 51,970

' In Great Britain October 7, 1947 7 Claims. (01. 235-61) This invention relates to arrangements for switching voltages and relates especially but not exclusively to such arrangements intended for use in electrical analogue computing apparatus.

In electrical analogue computing it is somenegative feedback whereby the amplifier oper-.

ates with a low input impedance, applying each voltage of the corresponding combination to the input electrode of the amplifier via a series feeding resistance, and by providing a switching device which can be operated to connect said load in the desired sequence to the output electrodes of the various amplifiers.

However, such a proposal requires as many amplifiers as there are voltage combinations and if the number of combinations is large the multiplicity of amplifiers can constitute a majordisadvantage. In addition if a break-before-make type of switch is provided, whereby when the switch is operated the load is disconnected from one amplifier before it is connected .to, the next, discontinuity results which cannot usually be tolerated. On the other hand if a make-beforebreak type of switch is provided, whereby the load is connected to the output of one amplifier before being disconnected from the output of the preceding amplifier then it is found-that at the instant when the load is connected simultaneously to two amplifiers the voltage set up across the load represents the summation of the voltage combination which has the more positive instantaneous value, which is also undesirable.

The object of the present invention is to provide an improved arrangement whereby a plurality of voltages can be switched successively across a load, with a view to avoiding or reducing the above described disadvantages According to the present invention there is provided an arrangement whereby a pluralityof voltages can be switched successively across a load, the arrangement comprising a thermionic" valve amplifier employing negative feedback whereby said amplifier operateswith a low input impedance and having said load coupled to the output, electrode of said amplifier, ,a switch having a plurality of input contacts which can be coupled respectively to sources ofsaid voltages and an output contact coupled to the input electrode of said amplifier, whereby said switch can be operated to couple said input contacts successively to said input electrode, and wherein said switch is such that on operation each input contact is switched into conductive connection with said output contact before the preceding input contact is switched from conductive connection therewith, and a separate negative feedback path is provided from said output electrode to each of said input contacts. 7 v

In most applications of the invention said feedback paths will be efiectively electrically identical whereby when said output contact is simultaneously in conductive connection with two input contacts the voltage set up across said load effectively represents the mean .of the voltages applied to said two input contacts. Inorder that the invention maybe clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawing, in which:

Figure 1 illustrates diagrammatically and partly in block form an arrangement according to one example of the invention, and

Figure 2 illustrates one application of the arrangement shown in Figure 1.

Referring to the drawing, the arrangement illustrated in Figure 1 is adapted to switch three voltages in sequence across a low impedance i5 but it will be understood that in practice any desired number of voltages can be switched by suitably modifying the arrangement. The arrangement comprises a high gain thermionic valve amplifier of any suitable constructionindicated in block form at I, having its input electrode (not shown) coupled to the input circuit of the arrangement .by a conductor 2, and having its output electrode coupled to the output load If:

by a conductor 3. The arrangement also comprises a switch of the make-before-break type, which may also be of anysuitable construction and is diagrammaticallyshown in the drawing as comprising a rotatable contact l connected to 8 to 3 a source of vo1tage E1, the contact t is connected in parallel to three voltage sources E2, E3 and E4 via feeding resistances 9, I and l I, while the contact I is connected in parallel to two voltage sources E and E6 via feeding resistances I2 and I3. A separate negative feedback path is provided from the output circuit of the amplifier I to each of the contacts 5 6, 1, in each case by means of resistances l4, the resistances l4 being of equal values. Negative feedback is therefore applied to the input electrode of the amplifier through one or other of the contacts 5, 6 or I irrespective of the position of the movable contact" 4, and moreover at the instants when the contact 4 engages two of the contacts 5, 6 and I negative feedback is provided to the input electrode, ,of the amplifier via two of the resistances I4. The negative feed.- back causes the amplifier to operate with a low input impedance, and when the contact 4 engages the contact 5 the voltage ,Eo set up across the load impedance I5 is proportional to E1, when the contact 4 engages the contact 6 the voltage E0 is proportional to E2+E3+E4, whereas when the contact 4 engages the contact 1 the voltage E0 is proportional to Es-l-Ee. When the contact simultaneously engages the two contacts 5 and 6, by reason of the fact that negative feedback is now provided via two resistances I I, the voltage E0 is proportional to whereas when the contact 4 simultaneously engages the two segments 6 and 1 the voltage E0 is proportional to Figure 2 illustrates an application of the arrangement of Figure 1 to apparatus for computing approximate values of a function of one independent variable, say I03), utilizing the Taylor expansion for the function. Corresponding parts in Figures 1 and 2 bear the same reference numerals, but the switch in Figure -2 has a numher say 100 of fixed segmental contacts 01, c2 Cr, only two denoted by the reference On and cn-l-l being however shown. Each of the contacts is connected to three sources of voltag the first voltage being proportional to the value of j(:1:) for a particular value of x, the second voltage being proportional to 1*(02) i. e., the first derivative of j(a:) for the same value of in, and the third voltage being proportional to f" (:c) for same value of :c, the values x1, x2 4:: being respectively allotted to the contacts 01, c2 Qt.

The values of x1, x2 $1 are moreover such that the difference between any two successive values is a constant, k say. Thus, as shown in Figure 2, the contact On is connected by feeding resistances I6, I! and I8 to three voltage. sourees I9, and 2! in which are stored voltages proportional to flit"), f(:cn) and For) respectively. Similarly the contact Cn+1 is connected by feeding resistances 22, 23 and 24 to three voltage sources 25, 25 and 21 in which are stored voltages proportional to j(:cn+i), f($ni- 1) and f(xn+1) and so on for the remaining fixed contacts. Each voltage source may be for example a potentiometer or an auto-transformer energized from a reference voltage and having the tap thereon located so that the voltage set up at the tap is representative of the desired particular value of fix), ,f'(:!:) or f"(a:) as the case may be. The rotatable contact 4 of the switch is arranged to be driven by suitable means indicated in block form at 28 whereby the movable contact 4 on operation of the device is rotated continuously in such a direction that it engages the contacts 01, ca Cr, in succession, and potentiometers 29, 30, 3| and 32, the taps on which are also driven by the aforesaid means 28 are interposed as shown between the sources 20, 2|, 26 and 2"! and the feeding resistances H, I8, 23 and 2d. The potentiometers 29 and 3| are linear law potentiometers of any suitable construction each arranged to multiply the voltage of the respective source 20 or 26 by an incremental factor AIL and the potentiometers 30 and 32 are square law potentiometers each arranged to multiply the voltage of the respective source 21 or 21 by the factor loam/g, t voltage to be multiplied being applied each case across the whole of the respective potentiometer so that the desired product is set up at the tap thereon. The taps on potentiometers 21 to 32 are driven synchronously with the contact 13 so that in the time interval when the contact 4 is in engagement with the contact (In the value of or for the potentiometers 29 and an varies uniformly from k/2 to +lc/2 and in the time interval when the contact 4 is in engagement with cn+1 the value of Ar for the potentiometers 3i and 32 varies uniformly over the same range of values. It will of course be appreciated that two potentiometers are provided for each of the contacts 01, c2 Cr, driven in a corresponding manner to that described for the case of the contacts on and aim.

Therefore, in the time interval when the contact 4 is in engagement with the contact an, for example, the voltage E0 set up across the load resistance 15 represents and this equals the first three terms of the Taylor expansion for fix) for x.:.-(:cn+A:c). Therefore in said last-mentioned time interval the voltage E0 varies uniformly from a value representing approximately f(xn.Ic/2) to a value representing approximately f(xn+k/2), and similarly in the time interval when the contact 3 is in engagement with the contact Cn, the voltage E0 varies uniformly from a value representing approximately j(:cn+1-k/2) to a value representing approximately f.(:tn+1+k/2). At the instant when the contact 4 engages both the contacts Cn and 011+; the voltage E0 represents t t is the v e E0 is the mean o wo n na 1y equal voltages so that effectively no discontinuity occurs in Eo as the contact 4 rotates and errors in the value of E0 will tend to be reduced. From the foregoing desoription of the operation Of the a ara us i r lat to th contacts on and +1 i w ll be nde st d that as the tact 4 engages the contacts 01, c2 Cr sequent a he o ta e E0 will v y u fo mly hrou h all the values which fir) takes for values of x the r n e from /,2 to e k/ Th a an ement of F gu e 2 an be a y applied to evaluating functions of more than one variable, the number of voltage sources connected to each of the switch contacts being then appropriately increased. The number of voltage sources connected to each contact can also be varied in dependence upon the accuracy required evaluating the function.

What I claim is:

El t ica an gue compu ing apparatus for computing approximate values of an arbitrary function (say j(:c)) of an independent variable (say :12) comprising a thermionic valve amplifier having an input circuit and an output circuit, a plurality of equispaced contacts corresponding respectively to successive particular values of :1:

separated from each other by the same interval Ax, a make-before-break selector memberforindividually switching said contacts to the input circuit of said amplifier, whereby said selector member maintains engagement with each of said contacts through a small range of displacement representative of the interval A0: and switches each contact to said input circuit before the previous contact is switched therefrom, means for feeding to each contact a current representative of the value of f(:r) at the particular value of a: corresponding to the contact, means for feeding to each contact, and in parallel with said lastmentioned current, a current representative of the product of a coeificient and the value of the first derivative of fur) atthe corresponding particular value of as, means operable concomitantly with said selector member to cause said coefficient to vary between the values Ax/2 and +Aac/2 as said selector member is displaced through the range in which it remains in engagement with each contact, and a separate feedback path from said output circuit to each of said contacts to cause said amplifier to operate as a current amplifier having a low input impedance compared with the impedance presented to said amplifier by said feeding means.

2. An arrangement for selecting different electrical signals, comprising a thermionic valve amplifier having an input circuit andv an output circuit, a plurality of input points, one for each signal, means for feeding to each input point a current representative of the respective signal, make-before-break means for individually switching said input points to the input circuit of said amplifier, whereby on switching from one input point to the next the latter point is switched to said input circuit before the previous point is switched therefrom, and a separate negative feedback path from the output circuit of said amplifier to each input point, whereby said feedback paths cause said amplifier to operate with a low input impedance compared with the impedance presented to said amplifier by said feeding means, and whereby negative feedback is provided by two feedback paths to reduce correspondingly the gain of the amplifier when two input points are simultaneously switched to said input circuit.

3. An arrangement for selecting different electrical signals, comprising a thermionic valve amplifier having an input circuit and an output circuit, a plurality of input points, one for each signal, means for feeding to each input point a current representative of the respective signal, make-before-break means for individually switching said input points to the input circuit of said amplifier, whereby on switching from one input point to the next the latter point is switched to said input circuit before the previous point is switched therefrom, and a separate negative feedback path from the output circuit of said amplifier to each input point, said feedback paths being effectively electrically identical, whereby said feedback paths cause said amplifier to operate with a low input impedance compared with the impedance presented to said amplifier by said feeding means, and whereby negative feedback is provided by two feedback paths to cause the output of said amplifier to represent when two input points are simultaneousl switched to said input circuit. a

4. An arrangement for selecting different electrical signals, comprising a thermionic valve amplifier having an input circuit and an output circuit, a make-before-break selector switch comprising a plurality of fixed contacts, one for each signal, and a make-before-break contact movable to selectively engage said fixed contacts and connected-to the input circuit of said amplifier for individually switching said fixed contacts to said input circuit, whereby on switching from one fixed contact to the next the latter fixed contact is switched to the input circuit before the previous fixed contact is switched therefrom, means for feeding to each fixed contact a current representative of the respective signal, and a separate negative feedback path from the output circuit of said amplifier to each fixed contact to cause said amplifier to operate with a low input impedance compared with the impedance presented to said amplifier by said feeding means, whereby negative feedback is provided by two feedback paths to reduce correspondingly the gain of the amplifier when twofixed contacts are simultaneously switched to said input circuit.

5. An arrangement for selecting different groups of electrical signals and for forming the sum of the signals in a selected group, comprising a thermionic valve amplifier having an input circuit and an output circuit, a plurality of input points, one for each group, means for feeding in parallel to each input point currents respresentative of the signals in the respective group,"make-before-break means for individually switching said input points to the input circuit of said amplifier, whereby on switching from one input point to the next the latter point is switched to said input circuit before the previous point is switched therefrom, and a separate negative feedback path from the output circuit of said amplifier to each input point to cause said amplifier to operate with a low input impedance compared with the impedance presented to the amplifier by said feeding means, whereby negative feedback is provided by the two corresponding feedback paths when two input points are simultaneously switched to said input circuit.

6. An arrangement for selecting different groups of electrical signals and for forming the sum of the signals in a selected group, comprising a thermionic valve amplifier having an input circuit and an output circuit, a makebefore-break selector switch having a plurality Of fixed contacts, one for each group, and a makebefore-break contact movable to selectively engage said fixed contacts and connected to the input circuit of said amplifier for individually switching said fixed contacts to said input circuit, whereby on switching from one fixed contact to the next the latter fixed contact is switched to said input circuit before the previous fixed contact is switched therefrom, means for feeding in parallel to each input contact currents representative of the signals in the respective group, and a separate negative feedback path from the output circuit of said amplifier to each input contact, whereby said amplifier operates with a low input impedance compared with the impedance presented to the amplifier by said feeding means, and whereby negative feedback is provided by the two corresponding feed back paths when two fixed contacts are simulta-i neously switched to' said input circuit.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Ripley May 26, 1942 Swartzel June 11, 1946 Giflen et a1. Apr. 12, 1949 Tulon May 24, 1949 Carpentier Dec. 6, 1949 

